It is important that motor drive systems in mobility vehicles are carefully controlled to avoid placing the user of the vehicles in danger and/or harming others around them, or at the very least to provide a stable and reliable driving experience. The mobility vehicles may be, for example, motorized wheelchairs or scooters that assist users in being mobile. These types of mobility vehicles may be termed “medical mobility vehicles” as they are generally used for persons who have a medical condition that reduces their ability to be self mobilised and so require a vehicle for them to move around.
The motor drive systems for this type of vehicle incorporate various different forms of motor control which aim to provide the user with reliable and safe operation of the vehicle. Improved control is particularly important for medical mobility vehicles due the reduced capability of users of these vehicles. In particular, improved speed control is desirable.
One problem associated with motor drive systems that incorporate permanent magnet or shunt wound motors is that changes in the armature resistance of the motors can cause changes in the speed of the mobility vehicle when the loading of the motor varies.
One form of control system used in mobility vehicles to compensate for the armature resistance effect is known as IXR load compensation. This traditional approach measures the load induced current of the drive motors and applies an extra voltage to compensate for the voltage lost across the motor armature resistance. This compensation method works throughout all four control quadrants of the mobility vehicle so that not only will the motor hold speed during positive loading it will also hold speed during negative loading or braking. The four control quadrants are known as forward drive, reverse braking, forward braking and reverse drive.
However, because of resistance effects, these motors may operate more slowly with imposed positive loading. Therefore, where tight speed regulation is important, as with drive systems for power wheelchairs and mobility scooters, accurate means of compensating for load induced speed changes is desirable. One such area where this is particularly relevant is that of medical mobility vehicles (such as power wheelchairs, for example) that use differential steering via two traction motors where steer precision is directly dependant on the speed holding ability of the motors.
The load compensation on these motors is not currently performed accurately because motor resistance is a variable that is a complex function of current, temperature, speed and time. If these inaccuracies lead to over-compensation, harsh and unstable driving characteristics may result. If under-compensated, driving precision may be compromised. Therefore, because of risks associated with over-compensation, it is generally required that under-compensation is performed.
Further, mobility vehicles traditionally need programming tools to adjust the level of compensation. For example, the level of compensation is adjusted only when the vehicle is taken in for a service.
One method of measuring motor resistance at stall is described in US Patent application US 2010/0007299. However, this system requires the motor not to be turning when the measurement is taken, which can result in inaccurate and varying measurements of resistance due to the large number of different positions in which the brush within the motor may stop. Further, this single resistance measurement remains fixed when the mobility vehicle is in use.
Another known method of compensating for motor resistance is described in U.S. Pat. No. 4,266,168. This describes a method of measuring the back EMF of a motor during use of the motor to generate a feedback signal that accounts for the motor's resistance. However, once measured, the feedback signal is only updated periodically resulting in a fixed feedback resistance value being applied between updates.
A further known problem with motor control in mobility vehicles is that of monitoring and detecting potential scenarios associated with a motor before they occur and reducing the potential for these scenarios to occur. In general, known systems that attempt to detect specific scenarios are complex and expensive. These scenarios may include, for example, potential damage to the motor, loss of performance, service requirements, etc.
For example, in a DC brush motor, problems may occur as the motor gets older when the brushes on the commutator start to wear. The brush damage can cause an increase in motor electrical resistance, making the motor more vulnerable to stalling and possible overheating.
Service scheduling of mobility vehicles is generally done by way of allocating a fixed period in between services. This can lead to increased damage to the motor components if wear and tear on the motor occurs rapidly in between services. Further, as wear and tear occurs in between services, the operation of the mobility vehicle may become erratic and unsafe.
Monitoring of motor temperature to prevent damage is generally performed by applying temperature sensors and sensing circuitry. However, this added circuitry and the extra components required increases cost and complexity.
The background discussion (including any potential prior art) is not to be taken as an admission of the common general knowledge.
An object of the present invention is to provide an improved method and control system for mitigating the effects of motor characteristics on the operation of a mobility vehicle.
A further object of the present invention is to provide an improved method and control system for updating a motor characteristic profile.
A further object of the present invention is to provide a method of detecting the likelihood of potential scenarios with a motor and providing an indication of these scenarios.
A further object of the present invention is to provide an improved method and system for controlling power applied to a motor dependent on one or more of the motor's performance characteristics.
A further object of the present invention is to provide an improved method and system for monitoring a motor based on one or more of the motor's performance characteristics.
Each object is to be read disjunctively with the object of at least providing the public with a useful choice.
The present invention aims to overcome, or at least alleviate, some or all of the afore-mentioned problems.